Erasure-proof development of electrostatic patterns



April 29, 1969 w. A. VAN DEN HEUVEL ETAL 3,441,425

ERASURE-PROOF DEVELOPMENT OF ELECTROSTATIC PATTERNS Filed May 18, 1964 Sheet of 4 0 A 4 49 cull ATTORNEY$ April 29, 1969 w, VAN DEN HEUVEL ET AL 3,441,426

ERASURE-PROOF DEVELOPMENT OF ELECTROSTATIC PATTERNS Filed May 18, 1964 Sheet 5 of 4 c iwm' lg v Ew ATTORNEYS United States Patent 292,922 Int. Cl. G03g 7/00 US. Cl. 11737 2 Claims ABSTRACT OF THE DISCLOSURE Erasure-proof images are produced on a photoconductive zinc oxide containing recording sheet by developing the electrostatic image on the sheet with an aqueous acidic liquid developer containing hexacyanoferrate (III) ions and a reducing compound selected from aromatic and hetrocyclic primary and secondary amines by which cationic dyes are formed upon oxidation. As the developer contacts the electrostatic image in the presence of the zinc ions in the sheet the amines are oxidized by the hexacyanoferrate (III) ions while the dyes are firmly absorbed to the for-med zinc hexacyanoferrates. Agents may be added to the developing solution to lower the surface tension and to slow the drying rate. The amines and the hexacyanoferrate (III) ions may be applied in two separate steps.

This invention relates to the erasure-proof development of electrostatic images.

It is known that an ink image prepared with an aqueous ink is poorly resistant to a treatment with water if no special precautions are taken. The production of an erasure-proof ink image with such inks thus is a problem that has received considerable attention in the art especially in connection with the detection of falsification and the improvement in storage stability of important documents.

The development of electrostatic latent images by means of liquid or ink is known for instance from the US. patent specification 2,551,582. It is mentioned therein that an electrostatic latent image can be developed with an electro-statically attractable finely divided liquid such as an ink mist. In the British patent specification 902,928 a method for liquid development of electrostatic charge patterns is described, whereby use is made of an electrically insulating liquid, wherein an aqueous phase has been emulsified, and wherein suspended pigment particles according to the principle of electrophoresis are deposited on the charged areas. According to the method for liquid development of electrostatic charge patterns, described in the Belgian patent specification 610,060, an electrostatic latent image is developed with a developing liquid, the interfacial tension of which in respect of the surface of a solid material carrying an electrostatic charge pattern is influenced by the magnitude and the sense of the electrostatic field strength on this surface in such a way, that this surface is wetted selectively or differently in correspondence with the electrostatic charge pattern.

Further, a method is described in the Belgian patent specification 625,335, whereby an electrostatic charge pattern is formed in or on a surface of a material comprising a photoconductive layer, by traversing this material by an electric field during or after the image-wise exposure, and whereby this surface is supplied simultaneously in a non-difierential way with a liquid, the surface tension ice of which with respect to said surface is influenced by the presence and the degree of the electrostatic charge in such a way, that the liquid wets the surface selectively or differentially according to the electrostatic charge pattern.

For the development of an electrostatic charge pattern colour reactions are described in the Belgian patent specification 610,060 between a compound in the developing liquid and a compound in the material to be developed parenthetically it may be noted that the formation of a visible image by the reaction of one or more compounds in the surface of the material to be developed with one or more compounds in a liquid phase possesses the advantage in that the formed image is thoroughly anchored in the surface of the material and hence is very resistant to mechanical rubbing off. These colour reactions can 'be activated or accelerated by heat and light, and by incorporating catalysts into the material that carries the electrostatic charge pattern and/or into the developing liquid.

It is an object of the present invention to produce visible erasure-proof prints 'by means of an aqueous ink onto an electrophotographic recording material that can provide zinc ions.

More particularly, it is an object of the present invention to provide liquid developing compositions for use in erasure-proof developmetnt of electrostatic charge patterns present in a recording element containing zinc oxide.

It is a further object of the present invention to provide several embodiments according to which erasure-proof images of an electrostatic image are produced, which is present on a photoconductive layer containing photoconductive zinc oxide.

The object of forming an erasure-proof image or record is accomplished according to one embodiment by imagewise or record-wise applying to a recording element, preferably a photoconductive zinc oxide containing electrophotographic element, which can provide Zinc ions, an aqueous composition (solution or dispersion) which in addition to hexacyanoferrate (III) ions contains an aromatic or heterocyclic primary or secondary amine, which occasionally contains a chromophoric group.

Under the influence of zinc ions provided by the recording element, the said amino compounds are quickly oxidized to dyes by hexacyanoferrate (I11) ions and these dyes are firmly adsorbed to the formed zinc hexacyanoferrates.

The said object is further accomplished according to a second embodiment by successively image-wise or recordwise applying to a recording element, preferably a photoconductive zinc oxide containing electrophotographic element, which can provide zinc ions, (a) an aqueous composition (solution or dispersion), which contains hexacyanoferrate (III) ions and (b) a cationic dye and/or a reducing agent.

The said object is further accomplished according to a third embodiment by sucessively image-wise or recordwise applying to a recording element, preferably a photoconductive zinc oxide containing electrophotographic element, which can provide zinc ions (a) an aqueous composition (solution or dispersion) which contains hexacyanoferrate (H) ions and (b) a cationic dye, e.g. a triarylmethane dye.

The reducing agent which according to the second embodiment can be applied together with the cationic dye, preferably is an aromatic or heterocyclic amine or a salt thereof.

According to an alternative of the second embodiment, a reducing agent may already be added to the liquid containing hexacyanoferrate (III) ions, e.g. an Oxidizable aromatic hydroxy compound of the hydroquinone type and/ or a leuco compound such as described in the patent 3 application No. 468,313 filed on the same date herewith.

It has been stated that in an aqueous liquid containing a cationic dye in addition to hexacyanoferrate (III) or hexacyanoferrate (II) ions, flocculation occurs, which is probably to be ascribed to the formation of an insoluble reaction product from the dye cations and the hexacyanoferrate (III) ions. Thus, a development liquid comprising the compounds mentioned is poorly suited for recording in a reproducible and erasure-proof way because of the scarce keepability.

Linking of the dye in an erasure-proof and waterproof way to the zinc oxide containing recording element is ascribed to the mordanting action of the zinc hexacyanoferrates formed in situ. During development of electrostatic charge patterns on a material comprising or supplying zinc ions in fact these materials are mordanted image-Wise and coloured on the mordanted areas. Mordanting is especially intensive when neutral or slightly acidic aqueous developing solutions are used comprising potassium hexacyanoferrate (II).

In order to dissolve the above-mentioned amines in the aqueous developing liquid containing hexacyanoferrate (III) ions, said amines are dissolved in their salt form because of their poor solubility in Water. Preferably the solution is maintained slightly acidic, since in the absence of zinc ions a low pH inhibits the oxidation of said amines.

For acidifying preferably strong acids are applied such as sulphuric acid and hydrochloric acid.

Amines which are considered for the application of the present invention are primary and secondary amines, the amino group of which making part of a conjugated system.

More especially good results are obtained with compounds comprising an aromatic system, which is substituted by at least one primary amino group, characterized thereby that at least one ortho-position in respect of the amino group is left unsubstituted, and that in other positions the aromatic system may be substituted by one or more substituents in such a way, that the sum of the a"- values of the further substituents preferably surpasses 0.66 and equals or is not larger than +0.874 (tr-values for Anilines H. R. Jalf, Chem. Rev. 53 (1953), p. 222, Table 7, and p. 225).

As examples of such primary amines are mentioned those corresponding to one of the formulae:

A represents a halogen atom such as chlorine or bromine, a lower alkyl radical such as methyl, an acetylamino group or an acetyl group, and

B represents a hydroxyl group or a primary amino group.

and

wherein 2-methyl-4-diethylaminoaniline, p-diethylaminoaniline, p-dimethylaminoaniline, 2,4,fi-triaminochlorobenzene, 2,4-dimethylaniline, 3,4-diaminophenol, 2,4-diaminophenol,

p-phenitidine,

veratrolamine, 2,5-dimethoxyaniline,

aniline,

a-naphthylamine,

benzidine, 4,4'-diaminodiphenylamine, and u-amino-fi-naphthol.

Heterocyclic primary amines which are considered for the preparation of erasure-proof ink prints are e.g.: 5- aminobenzothiazole, o-aminobenzothiazole and Z-aminobenzothiazole.

As secondary amines can be mentioned e.g. monomethylaniline and diphenylaniline. Over the non-coloured amines, the coloured amines have the advantage that a. richer assortment of brighter colours can be disposed of.

The hexacyanoferrate (III), which is used as an oxidizing agent for the amine preferably is potassium hexacyanoferrate (III).

During the oxidation of the said amines either dyes can be formed directly or colour couplers, which with an azo compound such as 2-aminoazobenzene form an azine dye.

The aqueous developing liquids which are used according to the present invention as a liquid ingredient comprise preferably water, although they may be mixed with other liquids or contain thicknening agents for adjusting the viscosity. The aqueous mordanting liquids preferably comprise 0.5 to 2% of potassium hexacyanoferrate (II) or potassium hexacyanoferrate (III). The aqueous developing compositions comprise preferably from 0.05 to 2% of the above-mentioned amines in addition to 0.5 to 2% of potassium hexacyanoferrate (III). The aqueous solutions of cationic dyes, applied to the areas, which are mordanted by potassium hexacyanoferrate (II) and/ or potassium hexacyanoferrate (III), may contain the mostly diversified types of cationic dyes, although triphenylmethane dyes, oxazine dyes and thiazine dyes are preferred. The developing liquids on the base of cationic dyes preferably comprise of from 0.3 to 20% of cationic dye.

Further all the developing liquids applied according to this invention may comprise 0.5 to 20% of a substance which influences the surface tension. This way, the surface tension can be lowered e.g. by the addition of watermiscible organic substances such as methanol, ethanol, acetone, methyl ethyl ketone, acetic acid, hydroquinone, lauryl sulphonates, dodecyl sulphontes, 'saponine and polyglycol derivatives.

Other usable surface-active substances are given in Textil Hilfsmittel und Waschrohstotfe by K. Lindner, Wissenschaftliche Verlagsgesellschaft m.b.H., 1954, Stuttgart.

Further, up to 10% of substances can be added to the developing liquid, which slow down the drying rate of the ink such as glycerol, glycol, and sorbitol, and from 0 to 10% of binding agents, which are soluble or dispersable in water such as gum arabic, carboxymethylcellulose, casein, poly(vinylpyrrolidone), poly(vinyl alcohol), poly- (vinyl acetate), polyacrylate, polystyrene, waxes, silicates and colloidal silicic acid.

After the oxidation of the above-mentioned amines, cationic dyes can still be adsorbed to those areas of a zinc oxide containing recording element, where potassium hexacyanoferrate (III) reacted with an amine, so that another colour can be given to a dye image, which is obtained by an oxidized amine.

Although in fact the method according to the present invention for obtaining erasure-proof ink prints and ink images can be used in the mostly diversified recording and developing techniques wherein zinc ions providing recording elements are used, this process in its different embodiments is especially suited for the use of liquid development techniques, which can be applied or which are applied to the development of electrostatic charge patterns onto layers contining zinc oxide, more particularly layers containing photoconductive zinc oxide dispersed in an insulating binding agent. In this case, the photoconductive zinc oxide supplies the zinc ions, which are necessary for the mordanting and colour-forming reactions.

According to a first preferred embodiment, a liquid A consisting of an aqueous solution of potassium hexacyano ferrate (II), is deposited in correspondence with an electrostatic charge pattern and according to one of the development techniques claimed in the Belgian patent specifications 610,060 and 625,335 to a photoconductive zinc oxide material with hydrophobic character. Hereby the charged areas of this material are mordanted, whereas the non-charged areas remain hydrophobic. Use is made of this selective hydrophobic/hydrophilic differentiation in order to wet the image-wise mordanted areas by means of an aqueous liquid B comprising a cationic dye. On the mordanted areas the cationic dye is strongly adsorbed and forms an erasure-proof dye image.

According to a second preferred embodiment, which can be applied to all known liquid development techniques for electrostatic charge patterns, a developing liquid C containing potassium hexacyanoferrate (III)v and one of the above-mentioned primary or secondary amines is deposited in correspondence with an electrostatic charge pattern onto a photoconductive material containing photoconductive zinc oxide. By the zinc ions formed in situ, the oxidation reaction of the amine with the potassium hexacyanoferrate (III) is accelerated in such a way, that immediately a strongly adsorbed dye image is generated.

The developing liquid C can also be employed e.g. in an electronic recording system described in the US. patent specification 2,577,894 according to which a modulated air or gas current is used, which is charged with ink particles.

According to a third preferred embodiment comprising elements of the two foregoing, a liquid image is formed by means of the developing liquid C according to any of the liquid development techniques claimed in the Belgian patent specifications 610,060 and 625,335, whereupon this liquid image is covered with an aqueous solution containing a cationic dye. This way, the dye image formed by oxidation can be covered in an erasure-proof way with a cationic dye, so as to yield colour patterns of the most diversified colour tones and possessing a high colour intensity.

By firstly applying image-wise the solution of hexacyanoferrate (III) ions to the recording material and thereafter occasionally a solution of a reducing agent, such as an aforementioned amine, together with a cationic dye, the possible oxidation of the reducing agent in the developing ink is excluded. Inversely, also first a solution can be applied of a reducing agent occasionally together with a cationic dye, and thereafter the solution containing the hexacyanoferrate (III) ions.

Of course, multicolour images can be obtained onto one single recording material by successive development with solutions comprising difierent amines, and occasionally by the subsequent application of aqueous compositions containing cationic dyes possessing a colour different from the colour of the dyes formed by oxidation of the amines. 0

The following examples illustrate the invention.

EXAMPLE 1 in ethanol 500 Ccs. 10% solution of monobutyl phosphate in ethanol 10 10% solution of succinic acid in dimethylformamide 10 1% solution of fluorescein (C.I. 45,350)

in ethanol 10 The photoconductive dispersion is applied to a paper support coated with aluminum foil in a ratio of 10 sq. m. per liter by means of a roller coating system and dried.

The photoconductive layer obtained after drying is charged by a corona to -300 v./cm. by a tension of 7000 v. at the corona wires, and then exposed for 0.7 see. through a diapositive with a 75-watt lamp placed at a distance of 10 cm. Then the latent image is developed with a developing apparatus as schematically shown in FIG. 1.

This apparatus comprises an ink roller 10 and a guiding roller 11. The ink roller 10 is moistened with ink from the ink bath 12. A doctor blade 13 regulates the amount of ink supplied. The guiding roller 11 being in contact with the support 15 presses the recording material 14 with its photoconductive layer 16 against the ink roller 10, which is wetted with ink. In the embodiment of the present example, the ink roller 10 is helieally grooved. The helical groove has a depth of 0.2 mm. and a top angle of 60. The recording material 14 is led between the guiding roller 11 and the ink roller 10 at a speed of 2 m./1nin.

The developing ink supplied in this way is prepared by mixing the following solutions A and B.

Solution A:

p-Eethoxyaniline hydrochloride g 1 Water ccs Solution B:

Potassium hexacyanoferrate (III) g 1 Water ccs 100 Potassium hexacyanoferrate (1H) g-.. 2 Picric acid g 2 Water ccs 100 When replacing the p-ethoxyaniline hydrochloride of solution A by one of the following amine hydrochlorides, the following dye images are obtained respectively:

2,5-dimethoxyaniline hydrochloride reddish-brown Veratrolamine hydrochloride Grey 3,4-diaminophenol hydrochloride Brown p-Dimethylaminoaniline hydrochloride Orange p-Diethylaminoaniline hydrochloride Purplish-blue EXAMPLE 2 A photoconductive layer as described in Example 1 is applied to a paper support with a resistivity of 10 ohmcm. After charging the layer as described in Example 1, an opaque original is episcopically projected for 30 sec. onto the photoconductive layer.

This projection occurs by means of a camera with a Schneider Kreuznach Xenar 4.4 lens having a focal length of 21 cm. and set at the diaphragm aperture 5.6.

The light-source consists of 2 Sylvania green 14-watt lamps. The length of the light beam from the light source to the original is 84 cm.

The development occurs as described in Example 1 but with a developing ink obtained by mixing a 2% aqueous solution of 2,4-diaminophenol hydrochloride with a 2% aqueous potassium hexacyanoferrate (III) solution.

A positive erasure-proof brown image is obtained.

Immediately after applying the said developing ink, the photoconductive layer is lead between a second pair of rollers. This pair of rollers likewise is mounted such as is shown schematically in FIG. 1 and described in Example 1. The ink roller, however, now is Wetted with a 1% aqueous solution of a cationic dye such as methyl ene blue, Thionin Violet (C.I. 52,000) or Brilliant Cresyl Blue (C.I. 51,010). Hereby the brown dye images are covered and blue erasure-proof images are obtained.

EXAMPLE 3 For the preparation of a photoconductive dispersion, the following products are mixed in a ball mill:

Plexigum P-25 (trade name for a solution of a polyacrylate resin in acetone, marketed by Rohm & Haas G.m.b.H., Darmstadt, Germany) The dispersion obtained is applied by dip-coating to an aluminium sheet so that the quantity of zinc oxide amounts to g./sq. m.

A latent electrostatic image is obtained on the photoconductive layer in the same way as in Example 1. Development occurs in an apparatus as schematically shown in FIG. 1. The guiding roller 11, however, is in this case an aluminium cylinder with a diameter of 15 mm. and a length of cm. The ink roller 10 is made of chrome nickel steel 18/8 and has a diameter of mm. and a length of 25 cm. The surface of the ink roller 10 is helically grooved in such a way that the windings touch each other. The groove is V-shaped with a Width and depth of 0.5 mm. The ink roller 10 freely rotates in the ink container 12 so that the groove is filled with ink. The ink is retained in the groove by capilla-rity.

The photoconductive material 14 is led between the ink roller 10 and the guiding roller 11 at a speed of 3 m./1min., the latent image facing the grooved ink roller 10.

The developing liquid is that of Example 1.

In order to increase the contrast between the exposed and the non-exposed areas, a -10 v. tension is applied between the guiding roller 11 and the ing roller 10.

EXAMPLE 4 To 500 ccs. of a 4% alcoholic solution of Flexbond D-13 (trade name), 150 g. of photoconductive Zinc oxide are added. Whilst thoroughly stirring, 10 cos. of a 10% aqueous solution of monobutyl phosphate are added.

This composition is ground for 24 hrs. in a ball mill and thereupon diluted with 500 cos. of a 4% alcoholic solution of the copolymer Flexbond D-13 (trade name) in order to obtain a dispersion. Before coating it, however, the following solutions are added successively whilst strongly stirring:

10% solution of succinic acid in dirnethylformamide ccs 10 0.1% solution of Rhodamine B (0.1. 45,170) in ethanol ccs 10 The photoconductive composition thus obtained is applied to a conductive paper support by dip-coating.

The photoconductive layer is charged till 400 v./cm. by means of a corona device with a tension of -6000 v. on the corona wires. Thereupon the charged photoconductive layer is exposed for 0.4 see. through a transparency with a lamp of 75 watts placed at a distance of 10 cm.

The obtained latent electrostatic image is developed in an apparatus such as illustrated schematically in FIG. 1.

In this apparatus the material to be developed is brought into contact with the mordanting liquid described hereinafter at a speed of 2 m./min. The roller 10 supplying this mordanting liquid is helically grooved. The helical groove possesses a depth of 0.2 mm. and a top angle of 60. The traveling speed of the photoconductive material 14 equals the peripheral speed of the roller 10, which supplies the mordanting liquid. The top portions of the grooved roller 10 come into contact with the photoconductive layer. The mordanting liquid is a 2% aqueous solution of potassium hexacyanoferrate (II).

Immediately after the application of said mordanting liquid, the photoconductive layer is led between a second pair of rollers, which is likewise mounted as is illustrated schematically in FIG. 1. The supply roller 10 as well as the guiding roller 11 possess a smooth surface.

The supply roller 10 brings the image-wise mordanted material into contact with a 1% aqueous solution 0f crystal violet. This cationic dye is firmly adsorbed to. the image-wise mordanted areas. An intensively coloured positive image is obtained.

EXAMPLE 5 A latent electrostatic image obtained as described in Example 1 is developed as described in that same example, with the difference, however, that the developing liquid is prepared by adding 100 cos. of a 2% aqueous solution of potassium hexacyanoferrate (III) to 100 cos. of an aqueous solution containing 2 g. of veratrolamine hydrochloride and 1 g. of Direct Black (C.I. 30,245)

(an azo dye containing free amine groups). A grey image.

is obtained.

EXAMPLE 6 The following photoconductive composition is applied to baryta-coated paper of g./sq. 111.:

Photoconductive zinc oxide g 37.5 Copoly(vinyl acetate/vinyl laurate) (/15) g 10 Ethanol n ccs 133 After drying, the thickness of the photoconductive layer amounts to 25 The photoconductive layer 16 of the recording material 18 is exposed for 1 min. into con-' tact with a negative original 19, as illustrated in FIG.-

2(a), by means of a lamp 20 of 75 Watts placed at'a distance of 10 cm. and then developed with an apparatus EXAMPLE 7 A latent electrostatic image is formed in a known way on a photoconductive selenium plate.

This latent electrostatic image is made visible by means of a 1% dispersion of p-phenitidine hydrochloride in hexane. The electrophoretically developed image is pressed against a paper the surface of which contains zinc oxide and which has previously been treated with a 2% aqueous solution of potassium hexacyanoferrate (III) of pH 2. An erasure-proof and purplish-blue transfer image is obtained on the paper.

EXAMPLE 8 A photoconductive composition such as prepared in example 4 is coated to a glassine paper support of 70 g./sq. m. in a proportion of 12 sq. m./l. Exposure and development are illustrated schematically in FIG. 3. Exposure occurs for 6 see. through a transparency 24 by means of a lamp 20 of 75 watts placed at a distance of cm. The inking roller 25 has a diameter of 39 mm., is helically grooved and made of chrome nickel steel 18/8. The helical groove 26 has a depth of 0.2 mm. and a top angle of 60.

The guiding roller 27 contacting the support possesses a diameter of 30 mm. and is made of aluminum. The inking roller 25 rotates at a speed of 100 revolutions/min. and forms a liquid head 28 against the photoconductive layer 16 to be developed. During develop ment, the inking roller 25 is connected to the positive pole and the guiding roller 27 is connected to the negative pole of a direct current source 29 of 200 v.

The developing ink is prepared by adding together a 2% aqueous solution of 2-methyl-7-aminobenzothiazole hydrochloride and a 2% aqueous solution of potassium hexacyanoferrate (HI).

A positive, erasure-proof and purplish-blue image 30 is obtained.

EXAMPLE 9 To 100 ccs. of a 10% solution of Staybelite Ester 10 (a glycerine ester of hydrogenated colophony marketed by The Hercules Powder Company Inc., Wilmington, Del., USA.) in methylene chloride, 2 ccs. of a 10% solution of monobutyl phosphate in ethanol and 30 g. of photoconductive zinc oxide powder are added. This mixture is ground for 4 hrs. in a ball mill. This photoconductive dispersion is coated to a paper support which is superficially closed by china clay, by means of dip-coating and in a proportion of g./sq. m. The photoconductive dispersion layer is dried at 40 C.

The obtained dry photoconductive layer is charged till 300 v./cm. by a corona discharge and then exposed through a transparency for 6 sec. by means of a lamp of 75 watts placed at a distance of 10 cm. The latent electrostatic image is developed with a developing apparatus such as schematically illustrated in FIG. 1 and described in Example 3.

The photoconductive layer carrying the latent image is brought into contact with a mordanting liquid consisting of a 2% aqueous solution of potassium hexacyanoferrate (IH) in said developing apparatus and at a speed of 2 m./rnin.

Immediately after applying said liquid, the photoconductive material is led between a second pair of rollers which is likewise mounted as indicated in FIG. 1. The supply roller brings the image-wise mordanted photoconductive layer into contact with a 1% aqueous solution of methylene blue. The cationic dye is firm-1y adsorbed to the mordanted areas and an intensively positive blue image is obtained.

EXAMPLE 10 To 100 ccs. of a 10% solution of Aroclor 5460 (trade name for a chlorinated polyphenyl resin marketed by Monsanto Chemicals Ltd., London) in methylene chloride, 2 ccs. of a 10% solution of monobutyl phosphate in ethanol, and 30 g. of photoconductive zinc oxide power are added. This mixture is ground in a ball mill for 4 hrs. and dip-coated to a baryta-coated paper support in a proportion of 20 g./sq. m. The photoconductive layer obtained after drying is charged, exposed image-wise and developed as described in Example 9 by means of a developing liquid containing 2 g. of 2,4-diaminophenol and 1 g. of potassium hexacyanoferrate (III) per 100 ccs. of water. Finally this material is subjected to an aftertreatment with a 2% aqueous solution of fuchsine, which is applied with the inking roller of the second pair of rollers.

EXAMPLE 11 To 100 ccs. of a 10% solution of Cumar Resin P- (trade name for a coumarone-indene resin marketed by Barrett Chemicals, New York, N.Y., USA.) in methylene chloride, 2 ccs. of a 10% soluton of monobutylphosphate in ethanol and g. of photoconductive zinc oxide powder are added. The whole mixture is ground for 4 hrs. in a ball mill. Just before coating, 4 ccslof a 5% solution of succinic acid in ethanol is added to the photoconductive dispersion. Thereupon the dispersion is coated onto glassine paper of 60 g./sq. m. in a proportion of 20 g./ sq. m.

After drying, the photoconductive zinc oxide layer is charged, exposed image-wise and developed such as described in Example 1, but with a developing liquid containing 2 g. of 3,4-dimethoxyaniline and 1 g. of potassium hexacyanoferrate (III) per 100 ccs. of water.

EXAMPLE 12 To a solution of 30 g. of Plexigum P-25 in 300 ccs. of toluene, g. of photoconductive zinc oxide and 6 ccs. of a 10% solution of monobutylphosphate in ethanol are added. This mixture is ground in a ball mill, whereupon 200 ccs. of toluene and 12 ccs. of a 5% solution of succinic acid in ethanol are added. The obtained photoconductive dispersion is coated onto parchemin paper in a proportion of 20 g./sq. m. and dried at 40 C.

After drying, the photoconductive layer into contact with a negative original 19 is exposed for 1 min. by means of a lamp 20 of 75 watts placed at a distance of 10 cm. and then developed in an apparatus as schematically illustrated in FIG. 4. The supply roller 25 is a cylinder made of chrome nickel steel 18/ 8 possessing a diameter of 25 mm. and a length of 25 cm. The surface of this roller is helically grooved. The grooves 26 possess V-shaped, and have a depth of 0.5 mm. and a top angle of 60. The grooves of this supply roller 25 are filled with a mordanting liquid 31 such as described in Example 4. Said liquid is held in the grooves by capillary forces. The guiding roller 27 is an aluminum cylinder with a diameter of 15 mm. and a length of 25 cm.

The exposed photoconductive material, its photoconductive layer 16 being in contact with the grooved roller 25 which is wetted by mordanting liquid 31, is led between this roller and the guiding roller 27 at a speed of 3 m./min.

During the traveling of the photoconductive material 18 carrying the latent image between both rollers, a direct tension of 400 v. is applied between both by connecting the guiding roller 27 with the positive pole and the supply roller 25 with the negative pole of a direct current Source 32.

After image-wise applying the mordanting liquid 31, the material is led between a pair of smooth rollers, between which, however, no tension has been applied and the supply roller of which has been Wetted with a 2% aqueous solution of Cresyl Blue (C.I. 51,010). A positive erasure-proof blue image is obtained.

The optical sensitization of the photoconductive layer is successfully carried out by means of 1 cc. of a 1% solution of fiuoresceine sodium salt (C.I. 45,350) in methyl glycol, by means of 4 ccs. of a 0.1 solution of Chrome Azurol S (C.I. 43,825) in methyl glycol, or by means of 1 cc. of a 0.1 solution of Rose Bengale (Cl. 45,440) in methyl glycol.

We claim:

1. A method of developing an electrostatic image on a recording element containing zinc oxide which comprises the steps of selectively applying to said image a developing liquid which is an aqueous developing composition consisting essentially of hexacyanoferrate (III) ions, a reducing compound of the group consisting of aromatic and heterocyclic primary and secondary amines, water and an acid compound to maintain the pH of the developing composition at 7 or below, the amount of said amino compound being sufiicient to produce a visible coloration when oxidized to form a cationic dye and the amount of said hexacyanoferrate (III) ions being sulncient to oxidize said amino compound to said dye.

2. The method of claim 1 wherein said developing composition contains about 0.52% of a water-soluble salt 1 l 12 of hexacyanoferrate (III) and about (LS-2% of said 3,285,741 11/ 1966 Ges'ierich et a1. 96-1 amino compound. 3,309,990 3/1967 Klupfel et a1. 96-1 X References Cited WILLIAM D. MARTIN, Primary Examiner. UNITED STATES PATENTS 5 E. J. CABIC, Assistant Examiner. 3,001,872 9/1961 Kurz 117-175 X 3,080,251 3/ 1963 Claus 117-175 US. Cl. X.R. 3,245,381 4/1966 Brenneisen et a1 96-1 252-621 

